Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session Q4: Inelastic Deformation VI: Strength |
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Chair: Rodney Clifton, Brown University Room: Hyatt Regency Constellation E |
Wednesday, August 3, 2005 9:30AM - 10:00AM |
Q4.00001: High-Pressure Strength Measurements Under Isentropic Loading Invited Speaker: Recent advances in magnetic loading techniques have permitted quasi-isentropes to be measured to unprecedented levels. However, the relevant equations for planar waves provide no information about transverse stresses, leaving the deviatoric (strength) behavior of an isentropically loaded material unknown. Because materials are much cooler under isentropic loading than under shock loading, they can remain solid and thus retain strength to very high pressures. Thus, to improve our ability to model material behavior under isentropic loading, techniques to measure strength are needed. In this paper, existing techniques for determining high-pressure strength will discussed along with their limitations. A technique for assessing the strength of isentropically loaded materials will be presented and used to determine the strength of an aluminum alloy using data from the Z machine and gas gun experiments. These results will be compared to existing models for material strength. Finally, limitations of the technique and future work needed will be discussed. \textit{Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.} [Preview Abstract] |
Wednesday, August 3, 2005 10:00AM - 10:15AM |
Q4.00002: Compressive strength of shocked aluminum for stresses of 4-22 GPa Hongfa Huang, James Asay Measurements of the compressive strength are presented for several aluminum alloys shocked to 22 GPa. Five well characterized alloys were studied, including 6061 with grain sizes of 50 $\mu $m, 30 $\mu $m and $<$5 $\mu $m, and both pure and ultra pure Al with 300 $\mu $m grain size. Yield strength was estimated using reshock and release techniques. These results show that quasi-elastic recompression occurs for all materials investigated and is independent of grain size and impurity level. The present data, together with other data, illustrate that the yield strength of Al increases with shock stress to 90 GPa and suggest that the increase in strength at shock states, $\Delta Y(\Delta Y=Y_{yield} -Y_{HEL} )$, increases with applied stress and plastic strain. A new model was developed to describe this increase and fits all data accurately to 55 GPa. The agreement strongly indicates that initial material properties, influence the ambient yield strength, but not the change in strength, which appears to be controlled by shock-deformation. [Preview Abstract] |
Wednesday, August 3, 2005 10:15AM - 10:30AM |
Q4.00003: More on the Strength of Materials Under High Shock Pressures Y. Ashuach, Z. Rosenberg, E. Dekel, A. Ginzberg One of the most important issues, in the field of dynamic loading of solids, is that of determining the strength of materials subjected to high shock pressures. Various techniques, most of them indirect, have been used to measure this property, by monitoring either stress or particle velocity in the shocked specimen. In this work we present a relatively simple and direct technique which is based on simultaneously measuring two stress histories, in a plate impact experiment with a structured target plate. The basic idea is to measure the stresses on the Hugoniot and release path simultaneously and to extract the strength under the high pressure from the difference between the two. Thus, the target is composed from two halves, one of which is a thick Plexiglas in which a manganin gauge is embedded some 2-4 mm from the impact face. The other half consists of the specimen backed by a thick Plexiglas, with another gauge at some distance into the plastic. The flyer material is a thick specimen disc. The two stress records correspond to the direct impact of the specimen on Plexiglas, resulting in a point on the Hugoniot of the specimen, and the second to a point on the unloading of the specimen. Results for different aluminum alloys, steel and alumina are presented. [Preview Abstract] |
Wednesday, August 3, 2005 10:30AM - 10:45AM |
Q4.00004: Lateral Stress Measurements in Pure Tungsten During Shock Loading J.C.F. Millett, N.K. Bourne, G.T. Gray Longitudinal and lateral stresses during the shock loading of pure tungsten have been measured using manganin stress gauges. The Hugoniot has been compared to the previous work of others and shown to be in close agreement. Lateral stresses have been shown to increase behind the shock front, implying that shear strength decreases. Whilst this is similar to results in tantalum, comparison with the recovery experiments of others suggests that this may be due to a brittle failure process. [Preview Abstract] |
Wednesday, August 3, 2005 10:45AM - 11:00AM |
Q4.00005: Polycrystal Modeling to Determine the Strengths of Shocked Ceramics Ruqiang Feng, Dongmei Zhang Voronoi polycrystal (VP) modeling coupled with finite element (FE) analysis has been developed to determine the effective shock strengths of polycrystalline ceramics. First, simulations with the 3-D VP-FE model and material parameters calibrated for $\alpha $-6H SiC are used to demonstrate that the mean stress behavior of a shocked ceramic is independent of heterogeneous incompressible flows at the grain level and can be accurately calculated if the crystal elasticity is accurately described and sufficient grains are used for statistical averaging. Next, a finite-strain nonlinear crystal elasticity model is worked out for (rhombohedral) $\alpha $-Al$_{2}$O$_{3}$ utilizing the available 2nd- and 3rd-order adiabatic elastic constants of the material. The VP-FE and nonlinear elasticity models are then used to compute the mean stress behavior of polycrystalline $\alpha $-Al$_{2}$O$_{3}$ for uniaxial compressions up to a longitudinal stress of 20 GPa. The results and the available longitudinal stress measurements give a determination of the material's effective shock strength, which is further modeled using the Drucker-Prager plasticity theory with a limiting strength cap. It is shown that the combination of strength model and mean stress calculation can well capture the key features observed for the shock response of the material: a Hugoniot elastic limit (HEL) of 9.1 GPa, a gradual post-HEL softening, and an effective strength saturation near twice the HEL. The effect of porosity and the issues related to polycrystal modeling are also discussed. [Preview Abstract] |
Wednesday, August 3, 2005 11:00AM - 11:15AM |
Q4.00006: Longitudinal and Lateral Stress Measurements in NiTi Under One-Dimensional Shock Loading Y. Meziere, J.C.F. Millett, N.K. Bourne, A. Wallwork, A. Workman Interest has been shown recently about the behavior at high strain rate of the shape memory alloy NiTi. However, the dynamic properties are not yet fully determined and the shear strength at under shock loading conditions has been little considered. This paper investigates the influence of the impact stress on the values of the shear stress under one-dimensional shock loading. The shear stress is defined by measuring the longitudinal and the lateral stresses. New data in terms of shock stress, particle velocity and shock velocity are gathered. The lateral stress was then determined using manganin stress gauge located inside the material. Results indicate that the lateral stress has a positive dependence upon the impact stress. A gradual decrease of the lateral stress was also observed while the longitudinal stress remains constant for the duration of the pulse length. This suggests that the shear strength increases behind the shock front. This also could reveal a complex mechanism of deformation behind the shock front during loading. [Preview Abstract] |
Wednesday, August 3, 2005 11:15AM - 11:30AM |
Q4.00007: ``Perturbation Method’’ For Study of Shear Strength of Materials at Pressures Up To $\sim$300gpa Aleksandr Lebedev Study of dependence of material shear strength on pressure is important scientific problem. The ``perturbation method'' is one of the basic methods allowing to perform these studies. This method is based on the fact that growth of initial perturbations depends on material strength and shapes of initial perturbations at intensive accelerations at interface between two media having different densities. Much data on shear strengths of materials at pressures of several tens GPa was obtained by this method in the planar geometry during X-ray recording of perturbation growth at boundary of plate accelerated by explosion products of condensed HE. To study strength at pressures of several hundred of GPa, the technique is suggested, where a cylindrical loading device is used. Perturbation growth is recorded on internal surface of shell, which compresses gas. Tests were performed for study of growth of artificial initial perturbations on internal surface of copper shell during its deceleration in the process of compression of the gas cavity in cylindrical device of high pressure. Calculated pressures in the shell during intensive deceleration are at the level of about 300GPa. The value of shear stresses in copper reached $\sim $25GPa. [Preview Abstract] |
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